For example, in the case where movie content or music content with a large dynamic range in audio volume is played back on a portable device with compact built-in speakers, not only does the audio volume become lower overall, but audio such as low-volume dialogue in particular becomes difficult to hear.
Thus, although normalization and automatic gain control technology does exist as technology for making the audio of such content easier to hear, volume control becomes audibly unstable unless data is read sufficiently far enough ahead.
There also exists technology that boosts low-volume portions and compresses high-volume portions of audio by means of a dynamic range compression process for volume. However, with a compression process, it is difficult to obtain large audio enhancement effects if generalized volume boost and compression settings are used. In order to obtain large effects, it is necessary to vary settings on a per-content basis.
For example, there exists technology that takes a sound pressure level specified by dialogue normalization as a basis, boosting signals with a lower sound pressure level and compressing signals with a higher sound pressure level. With this technology, however, it is necessary to specify boost and compression settings and a sound pressure level for dialogue normalization at the time of encoding the audio signal in order to obtain sufficient effects.
Furthermore, technology has also been proposed in which, in the case of compressing the dynamic range of audio volume, faint sounds in an audio signal are made easier to hear by multiplying the audio signal by coefficients determined by the average of the absolute values of the audio signal (see PTL 1, for example).